Organic light emitting devices have been shown to have sufficient brightness, range of color and operating lifetimes for use as a practical alternative technology to LCD-based full color flat-panel displays (S. R. Forrest, P. E. Burrows and M. E. Thompson, Laser Focus World, February 1995). Furthermore, since many of the organic thin films used in such devices are transparent in the visible spectral region, they potentially allow for the realization of a completely new type of display pixel in which the red (R), green (G), and blue (B) emission layers are placed in a vertically stacked geometry to provide a simple fabrication process, minimum R-G-B pixel size, and maximum fill factor.
It is believed herein that ideas which have been disclosed for using separate, side-by-side R, G, B OLEDs to make a full color display (C. W. Tang et al, U.S. Pat. No. 5,294,869 (1994)) have not been realized in a practical device.
Such schemes suffer from a complex layer structure, and lack of known methods for damage-free, post-deposition, patterning of organic layers at the resolution required for color displays. Others have suggested using an array of white OLEDs (J. Kido, M. Kimura, K. Nagai, Science, vol. 267, 1332 (1995)) backed by side-by-side R, G and B color filters deposited and patterned prior to OLED fabrication. However, such a design sacrifices at least 66% of the light from each white OLED, with the remainder being absorbed in the filter generating heat. Such a design suffers, therefore, from low efficiency and accelerated degradation. Alternative schemes based on microcavity filtering of a broad OLED spectrum (A. Dodabalapur, L. J. Rothberg, T. M. Miller and E. W. Kwock, Appl. Phys. Lett., vol. 64, 2486 (1994)) suffer from complex and expensive substrate patterning requirements and directionality of the resulting color.
Published examples of tunable OLEDs utilize a blend of either two polymers (M. Granstrom and O. Inganas, Appl. Phys. Lett., vol. 68, 147 (1996)) or a polymer doped with semiconductor nanocrystallites (B. O. Dabbousi, M. G. Bawendi, O. Onitsuka and M. F. Rubner, Appl. Phys. Lett., vol. 66, 1316 (1995); V. L. Colvin, M. C. Schlamp, A. P. Allvisatos, Nature 370, 354 (1994)). Each component of the blend emits radiation having a different spectral energy distribution. The color is tuned by varying the applied voltage. A higher voltage results in more emission from the higher bandgap polymer, which emits radiation toward the blue region of the spectrum, while also resulting in higher overall brightness due to increased current injection into the device. Although tuning from orange to white has been demonstrated, incomplete quenching of the low-energy spectral emission appears to prohibit tuning completely into the blue. In addition, emission intensity can only be controlled by using pulsed current and reduced duty cycles. In a color display, therefore, prohibitively high drive voltages and very low duty cycles may be necessary for blue pixels. This necessitates a complex driver circuit, renders passive matrix operation extremely difficult, if not impossible, and is likely to accelerate degradation of the display.
A transparent organic light emitting device (TOLED) which represents a first step toward realizing high resolution, independently addressable stacked R-G-B pixels has been reported recently in International Patent Application No. PCT/US95/15790 which corresponds to co-pending U.S. Ser. No. 08/354,674. This TOLED had greater than 71% transparency when turned off and emitted light from both top and bottom device surfaces with high efficiency (approaching 1% quantum efficiency) when the device was turned on. The TOLED used transparent indium tin oxide (ITO) as the hole-injecting electrode and a Mg--Ag-ITO layer for electron-injection. A device was disclosed in which the Mg--Ag-ITO electrode was used as a hole-injecting contact for a second, different color-emitting OLED stacked on top of the TOLED. Each device in the stack (SOLED) was independently addressable and emitted its own characteristic color through the transparent organic layers, the transparent contacts and the glass substrate, allowing the entire device area to emit any combination of color that could be produced by varying the relative output of the two color-emitting layers. Thus, for the specific device disclosed in PCT/US95/15790, which included a red-emitting layer and a blue-emitting layer, the color output produced by the pixel could be varied in color from deep red through blue.
It is believed that publication of PCT/US95/15790 provided the first disclosure of an integrated OLED where both intensity and color could be independently varied by using external current sources. As such, PCT/US95/15790 represents the first proof-of principle for achieving integrated, full color pixels which provide the highest possible image resolution, which is due to the compact pixel size, and low cost fabrication which is due to the elimination of the need for side-by-side growth of the different color-producing pixels.
In building OLEDs, several different layers are typically used which have very different physical properties so that the charge carriers (holes and electrons) can be trapped in the organic material and then recombine there, leading to exciton formation and light emission. Although a large number of different materials have been identified that function as hole transporters, relatively few electron transporting materials have been reported. It would be desirable if a larger number of improved electron transporting materials were available that could be used in OLEDs. In particular, it would be desirable to have electron transporting materials having improved carrier mobility and carrier density and which can be readily prepared from air stable precursors. It would be further desirable to have electron transporting materials that are selected from a family of compounds that can also serve as emissive materials having a spectral emission that can be adjusted for use in an OLED.